Golf ball

ABSTRACT

The invention provides a golf ball having a core and a cover of at least one layer. The cover includes an outermost layer and at least one intermediate layer interposed between the outermost layer and the core. The core has a cross-sectional hardness which, letting R (mm) be the radius of the core, A be the JIS-C hardness at the center of the core, B be the JIS-C hardness at a position R/3 mm from the core center, C be the JIS-C hardness at a position R/1.8 mm from the core center, D be the JIS-C hardness at a position R/1.5 mm from the core center, and E be the JIS-C hardness at the surface of the core, satisfies formulas (1) B−A&lt;D−B, (2) C−A≦8, and (3) E−B≧15. The intermediate layer and the outermost layer have a hardness relationship therebetween which satisfies the formula “material hardness of intermediate layer&lt;material hardness of outermost layer.”

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 13/461,187 filed on May 1, 2012, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball having a core and a cover of at least one layer, which cover includes an outermost layer and at least one intermediate layer interposed between the outermost layer and the core. More specifically, the invention relates to a golf ball which achieves satisfactory distances both on shots with drivers and on shots with middle irons used by professional golfers and skilled amateurs, and which also has an excellent durability to cracking when repeatedly struck.

Solid golf balls have a relatively simple inner structure composed of a core and a cover, although various golf ball properties such as rebound, feel, spin on approach shots and durability depend to a large degree on synergy between the core and the cover. In numerous disclosures to date, efforts have been made to optimize the rebound and feel of the ball, and also the spin rate of the ball on approach shots, by specifying in detail the cross-sectional hardness of the core. Such art is described in, for example, the following technical literature: JP-A 2011-136020 (and the corresponding published U.S. Patent Application No. 2011/0159999), JP-A 2011-136021 (and the corresponding published U.S. Patent Application No. 2011/0159998), JP-A 2007-152090 (and the corresponding U.S. Pat. No. 7,273,425), JP-A 2008-194473 (and the corresponding U.S. Pat. No. 7,481,722), and JP-A 2010-214105 (and the corresponding U.S. Pat. No. 7,909,710).

In addition, numerous disclosures have been made on art which, in rubber compositions for golf ball cores, specifies the core hardness profile from the standpoint of the rubber formulation. For example, JP-A 2006-312044 (and the corresponding published U.S. Pat. No. 7,278,929) discloses art in which powdered sulfur is added so as to increase the hardness difference between the center and surface of the core to at least a given value. Moreover, technical documents in which rubber formulations have been designed to achieve a desired core hardness profile include, for example, JP-A 2006-167452 (and the corresponding U.S. Pat. No. 7,276,560), JP-A 2006-289074 (and the corresponding U.S. Pat. No. 7,381,776), JP-A 2002-000765 (and the corresponding U.S. Pat. No. 6,679,791), JP-A 2010-188199 (and the corresponding U.S. Pat. No. 6,679,791, which is the same as the preceding), JP-A 2007-167257, JP-A 2008-68077 (and the corresponding U.S. Pat. No. 7,335,115), and JP-A 2008-119461 (and the corresponding U.S. Pat. No. 7,300,362).

However, although a certain degree of improvement can be expected with regard to conventional rubber formulations and core hardness profiles, even further increases in distance and improvements in durability are desired. Lately, research and development on golf balls has been especially intense. Improvements in the overall properties of the golf ball have been desired in order to secure a competitive advantage with the ball.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball which achieves satisfactory distances both on shots with drivers and on shots with middle irons used by professional golfers and skilled amateurs, and which also has an excellent durability to cracking when repeatedly struck.

As a result of extensive investigations aimed at achieving the above objects, the inventor has developed golf ball cores having cross-sectional hardnesses like those shown in FIGS. 3 and 4 of working examples of the invention; that is, cores which, in the cross-sectional hardness profiles thereof, have a cross-sectional hardness from the core center to a position at a given distance therefrom that is relatively soft and free of large fluctuations in hardness, and have a cross-sectional hardness thereafter up to the core surface that rises at a steep gradient. The inventor has found that golf balls having a core with such a cross-sectional hardness achieve in particular satisfactory distances, both on shots with drivers and on shots with middle irons such as a number six iron (I#6) used by professional golfers and skilled amateurs, and also exhibit an improved durability to cracking when repeatedly struck.

Among recent golf balls in particular, three-piece solid golf balls and four-piece solid golf balls featuring a urethane cover are widely used by professional golfers and skilled amateurs. The present invention, by optimizing the internal hardness profile of the core in the above ball, improves not only the distance achieved on shots with drivers used by professionals and skilled amateurs, but also the distance achieved with middle irons such as a number six iron (I#6) used by such golfers. Moreover, the invention provides a ball which, along with improving the distance performance, has an excellent durability to cracking when repeatedly struck and is capable of withstanding harsh conditions of use.

Accordingly, the invention provides the following golf ball.

[1] A golf ball comprising a core and a cover of at least one layer, wherein the cover comprises an outermost layer and at least one intermediate layer interposed between the outermost layer and the core; the core has a cross-sectional hardness which, letting R (mm) be a radius of the core, A be a JIS-C hardness at a center of the core, B be a JIS-C hardness at a position R/3 mm from the core center, C be a JIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-C hardness at a position R/1.5 mm from the core center, and E be a JIS-C hardness at a surface of the core, satisfies formulas (1) to (3) below:

B−A<D−B  (1)

C−A≦8  (2)

E−B≧15; and  (3)

the intermediate layer and the outermost layer have a hardness relationship therebetween which satisfies the following formula:

material hardness of intermediate layer<material hardness of outermost layer.

[2] The golf ball of [1], wherein at least one intermediate layer is formed of a material obtained by blending:

an ionomer resin component comprising (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer mixed with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and

(e) a non-ionomeric thermoplastic elastomer in a weight ratio between 100:0 and 50:50.

[3] The golf ball of [1], wherein at least one intermediate layer is formed of a material obtained by blending as essential ingredients:

100 parts by weight of a resin component composed of, in admixture,

-   -   a base resin of (a) an olefin-unsaturated carboxylic acid random         copolymer and/or a metal ion neutralization product of an         olefin-unsaturated carboxylic acid random copolymer mixed         with (b) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester random terpolymer and/or a metal ion         neutralization product of an olefin-unsaturated carboxylic         acid-unsaturated carboxylic acid ester random terpolymer in a         weight ratio between 100:0 and 0:100, and     -   (e) a non-ionomeric thermoplastic elastomer in a weight ratio         between 100:0 and 50:50;

(c) from 5 to 120 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and

(d) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c).

[4] The golf ball of [1], wherein the ball has a deflection, when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf), of from 10 to 13 mm. [5] The golf ball of [1], wherein the core is formed of a rubber composition which includes an organic peroxide having a one-minute half-life temperature of from about 165° C. to about 185° C. and sulfur. [6] The golf ball of [1], wherein the ball has, at a surface of a sphere composed of the core encased by the intermediate layer, a JIS-C hardness of not more than 90. [7] The golf ball of [1], wherein the outermost layer of the cover includes a base resin comprising an ionomer having a material hardness (Shore D) of from 50 to 70. [8] The golf ball of [1], wherein the core has a diameter of from 32 to 41 mm. [9] The golf ball of [1], wherein the core is composed of a single layer.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing a golf ball according to one embodiment of the invention.

FIG. 2 is a plan view showing the dimples used in the working examples of the invention and the comparative examples.

FIG. 3 is a graph showing the cross-sectional hardness profile of the core used in Example 1.

FIG. 4 is a graph showing the cross-sectional hardness profile of the core used in Example 2.

FIG. 5 is a graph showing the cross-sectional hardness profile of the core used in Comparative Example 1.

FIG. 6 is a graph showing the cross-sectional hardness profile of the core used in Comparative Example 2.

FIG. 7 is a graph showing the cross-sectional hardness profile of the core used in Comparative Example 3.

FIG. 8 is a graph showing the cross-sectional hardness profile of the core used in Comparative Example 4.

FIG. 9 is a graph showing the cross-sectional hardness profile of the core used in Comparative Example 5.

FIG. 10 is a graph showing the cross-sectional hardness profile of the core used in Comparative Example 6.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described more fully below.

The golf ball of the invention has a construction which includes a core and a cover of at least one layer. As mentioned above, the core may be composed of a single layer or may be formed of a plurality of two or more layers, although it is preferable in this invention for the core to be composed of a single layer. Also, in this invention, the “cover” refers collectively to whatever layers are formed to the outside of the core, and is composed of at least one layer. The cover includes an outermost layer and at least one intermediate layer interposed between the outermost layer and the core. Accordingly, in cases where the intermediate layer is a single layer, the cover is composed of two layers: one intermediate layer (the first intermediate layer) and an outermost layer. It is possible to additionally provide a second intermediate layer between the core and the above intermediate layer (first intermediate layer), in which case the cover has a three-layer construction composed of two intermediate layers and an outermost layer. Also, a large number of dimples are generally formed on the outside surface of the outermost layer of the cover. In cases where there are a plurality of intermediate layers, the intermediate layer positioned on the innermost side and adjacent to the core is sometimes referred to in particular as the “envelope layer.”

The core used in the invention is described. This core may be obtained by vulcanizing a rubber composition composed primarily of a rubber material. No particular limitation is imposed on the rubber composition. In a preferred embodiment, the core may be formed using a rubber composition containing, for example, a base rubber, a co-crosslinking agent, a crosslinking initiator, sulfur, an organosulfur compound, a filler and an antioxidant. Moreover, it is preferable to use polybutadiene as the base rubber of this rubber composition.

The polybutadiene must be one having a cis-1,4 bond content of at least 60% (here and below, “%” refers to percent by weight), preferably at least 80%, more preferably at least 90%, and most preferably at least 95%. If the cis-1,4 bond content is too low, the resilience decreases. In addition, the polybutadiene has a 1,2-vinyl bond content of preferably not more than 2%, more preferably not more than 1.7%, and even more preferably not more than 1.5%.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably at least 30, and more preferably at least 35, with the upper limit being preferably not more than 100, and more preferably not more than 90.

The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity (JIS K6300) as measured with a Mooney viscometer, which is a type of rotary plastometer. This value is represented by the unit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity, “L” stands for large rotor (L-type), and “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes. The “100° C.” indicates that measurement was carried out at a temperature of 100° C.

From the standpoint of obtaining a molded and vulcanized rubber composition having a good resilience, it is preferable for the polybutadiene to be a polybutadiene synthesized with a rare-earth catalyst or a group VIII metal compound catalyst.

The rare-earth catalyst is not subject to any particular limitation, although preferred use may be made of a lanthanum series rare-earth compound. Also, where necessary, an organoaluminum compound, an alumoxane, a halogen-bearing compound and a Lewis base may be used in combination with the lanthanum-series rare-earth compound. Preferred use may be made of, as the various foregoing compounds, those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

Of such rare-earth catalysts, the use of a catalyst which employs any of the lanthanum series rare-earth elements neodymium, samarium and gadolinium is preferred, with the use of a neodymium catalyst being especially recommended. In such cases, a polybutadiene rubber having a high 1,4-cis bond content and a low 1,2-vinyl bond content can be obtained at an excellent polymerization activity.

The polybutadiene has a molecular weight distribution Mw/Mn (where “Mw” stands for weight-average molecular weight, and “Mn” stands for number-average molecular weight) of preferably at least 1.0, and more preferably at least 1.3. The upper limit is preferably not more than 6.0, and more preferably not more than 5.0. If Mw/Mn is too small, the workability may decrease, whereas if it is too large, the resilience may decline.

The above polybutadiene is used as the base rubber, in which case the proportion of the polybutadiene within the rubber as a whole is preferably at least 40 wt %, more preferably at least 60 wt %, even more preferably at least 80 wt %, and most preferably at least 90 wt %. The above polybutadiene may account for 100 wt %, preferably 98 wt % or less, and even more preferably 95 wt % or less, of the base rubber.

Examples of cis-1,4-polybutadiene rubber that may be used include the high-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 and BR51 available from JSR Corporation.

Rubber components other than the above-described polybutadiene may also be included in the base rubber, insofar as the objects of the invention can be achieved. Illustrative examples of such other rubber components include polybutadienes other than the above polybutadiene, and other diene rubbers, such as styrene-butadiene rubbers, natural rubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

The co-crosslinking agent is not subject to any particular limitation in this invention. Illustrative examples include unsaturated carboxylic acids, and the metal salts of unsaturated carboxylic acids. Examples of suitable unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. The use of acrylic acid or methacrylic acid is especially preferred. The metal salts of unsaturated carboxylic acids are exemplified by the above unsaturated carboxylic acids which have been neutralized with a desired metal ion. Illustrative examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred. The content of these unsaturated carboxylic acids and/or metal salts thereof per 100 parts by weight of the base rubber is preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 20 parts by weight. The upper limit is preferably not more than 45 parts by weight, more preferably not more than 43 parts by weight, and even more preferably not more than 41 parts by weight.

An organic peroxide is preferably used as the crosslinking initiator. Specifically, the use of an organic peroxide having a relatively high thermal decomposition temperature is preferred. For example, an organic peroxide having an elevated one-minute half-life temperature of from about 165° C. to about 185° C., such as a dialkyl peroxide, may be used. Illustrative examples of dialkyl peroxides include dicumyl peroxide (“Percumyl D,” from NOF Corporation), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (“Perhexa 25B,” from NOF Corporation), and di(2-t-butylperoxyisopropyl)benzene (“Perbutyl P,” from NOF Corporation). Preferred use can be made of dicumyl peroxide. These may be used singly or two or more may be used in combination. The half-life is one indicator of the organic peroxide decomposition rate, and is expressed as the time required for the original organic peroxide to decompose and the active oxygen content therein to fall to one-half. The vulcanization temperature for the core-forming rubber composition is generally in a range of from 120 to 190° C. Within this range, the thermal decomposition of high-temperature organic peroxides having a one-minute half-life temperature of about 165° C. to about 185° C. is relatively slow. With the rubber composition of the invention, by regulating the amount of free radicals generated, which increases as the vulcanization time elapses, a crosslinked rubber core having a specific internal hardness profile is obtained.

The crosslinking initiator is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 0.1 part by weight, more preferably at least 0.2 part by weight, and even more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5.0 parts by weight, more preferably not more than 4.0 parts by weight, even more preferably not more than 3.0 parts by weight, and most preferably not more than 2.0 parts by weight. Including too much may make the core too hard, resulting in an unpleasant feel at impact, and may markedly lower the durability to cracking. On the other hand, if the amount included is too small, the core may become too soft, resulting in an unpleasant feel at impact and markedly lowering productivity.

Fillers that may be preferably used include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or two or more may be used in combination. The amount of filler included per 100 parts by weight of the base rubber may be set to preferably at least 1 part by weight, and more preferably at least 3 parts by weight. The upper limit in the amount included per 100 parts by weight of the base rubber may be set to preferably not more than 200 parts by weight, more preferably not more than 150 parts by weight, and even more preferably not more than 100 parts by weight. At a filler content which is too high or too low, a proper weight and a suitable rebound may be impossible to obtain.

Antioxidants that may be used include, for example, commercial products such as Nocrac NS-6, Nocrac NS-30 and Nocrac 200 (all products of Ouchi Shinko Chemical Industry Co., Ltd.). These may be used singly, or two or more may be used in combination.

The amount of antioxidant included per 100 parts by weight of the base rubber, although not subject to any particular limitation, is preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The upper limit is preferably not more than 1.0 part by weight, more preferably not more than 0.7 part by weight, and even more preferably not more than 0.4 part by weight. If the antioxidant content is too high or too low, a suitable core hardness gradient may not be obtained, as a result of which it may not be possible to obtain a good rebound, durability, and spin rate-lowering effect on full shots.

Sulfur may be optionally included in the rubber composition. The sulfur is exemplified by the product available from Tsurumi Chemical Industry Co., Ltd. under the trade name “Sulfax-5.” The amount of sulfur included can be set to more than 0, and may be set to preferably at least 0.005 part by weight, and more preferably at least 0.01 part by weight, per 100 parts by weight of the base rubber. The upper limit in the amount of sulfur, although not subject to any particular limitation, may be set to preferably not more than 0.5 part by weight, more preferably not more than 0.4 part by weight, and even more preferably not more than 0.1 part by weight. By adding sulfur, hardness differences in the core can be increased. However, adding too much sulfur may result in undesirable effects during hot molding, such as explosion of the rubber composition, or may considerably lower the rebound.

In addition, an organosulfur compound may be included in the rubber composition so as to impart an excellent rebound. Thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof are recommended for this purpose. Illustrative examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, and the zinc salt of pentachlorothiophenol; and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The use of diphenyldisulfide or the zinc salt of pentachlorothiophenol is especially preferred.

The amount of the organosulfur compound included per 100 parts by weight of the base rubber is at least 0.05 part by weight, preferably at least 0.07 part by weight, and more preferably at least 0.1 part by weight. The upper limit is not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Including too much organosulfur compound may excessively lower the hardness, whereas including too little is unlikely to improve the rebound.

The core can be produced by vulcanizing and curing the rubber composition containing the various above ingredients. For example, production may be carried out by using a mixing apparatus such as a Banbury mixer or a roll mill to mix the ingredients, carrying out compression molding or injection molding using a core-forming mold, then suitably heating, and thereby curing, the molded body at a temperature sufficient for the organic peroxide and the co-crosslinking agent to act, such as from about 100° C. to about 200° C., for a period of 10 to 40 minutes.

The core diameter, although not subject to any particular limitation, is preferably at least 32 mm, and more preferably at least 33 mm. The upper limit is preferably not more than 41 mm, and more preferably not more than 40 mm. At a core diameter outside of this range, the ball's durability to cracking may dramatically decline, or the initial velocity of the ball may decrease.

It is critical for the above core to have a cross-sectional hardness which, letting R (mm) be a radius of the core, A be a JIS-C hardness at a center of the core, B be a JIS-C hardness at a position R/3 mm from the core center, C be a JIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-C hardness at a position R/1.5 mm from the core center, and E be a JIS-C hardness at a surface of the core, satisfies formulas (1) to (3) below:

B−A<D−B  (1)

C−A≦8  (2)

E−B≧15.  (3)

By adjusting the cross-sectional hardness of the core so as to satisfy formulas (1) to (3), that is, conceptually, by finishing the core to a hardness profile in which the cross-sectional hardness from the core center to a position at a given distance therefrom is relatively soft and free of large hardness fluctuations and in which the cross-sectional hardness thereafter up to the core surface rises at a steep gradient, it is possible to suppress excessive deformation of the core on full shots with a driver or a middle iron and thus optimize core deformation, and to suppress a loss of initial velocity and an increase in the spin rate. As a result, satisfactory distances can be obtained on shots with a driver and on shots with a middle iron such as a I#6 used by professional golfers and skilled amateurs, in addition to which the durability to cracking on repeated impact can be improved.

In above formula (1), if the B−A value is larger than the D−B value, a sufficient spin rate-lowering effect is not obtained, as a result of which the desired distance is not achieved. The B−A value has an upper limit of preferably not more than 7, more preferably not more than 6, and even more preferably not more than 5, and a lower limit of preferably at least −4, more preferably at least −3, and even more preferably at least −2.

Also, in above formula (2), if the C−A value is too large, a sufficient spin rate-lowering effect is not obtained, as a result of which the desired distance is not achieved. The C−A value has an upper limit of preferably not more than 7, and more preferably not more than 6, and a lower limit of preferably at least 0, and more preferably at least 1.

Moreover, in above formula (3), if the E−B value is too small, a sufficient spin rate-lowering effect is not obtained, as a result of which the intended distance is not achieved. The E−B value has a lower limit of preferably at least 15, and an upper limit of preferably not more than 40, and more preferably not more than 38.

The value E−A, which is the JIS-C hardness difference between the surface of the core and the center of the core, has a lower limit of preferably at least 10, more preferably at least 12, and even more preferably at least 14, and an upper limit of preferably not more than 45, more preferably not more than 40, and even more preferably not more than 35. If the E−A value is too large, a good initial velocity may not be obtained and the durability may worsen. On the other hand, if the E−A value is too small, the spin rate may rise excessively, resulting in a poor distance, and the feel at impact may harden. The JIS-C hardness at the core center, although not subject to any particular limitation, has a minimum value of preferably at least 50, more preferably at least 52, and even more preferably at least 54, and a maximum value of preferably not more than 70, more preferably not more than 68, and even more preferably not more than 66. The JIS-C hardness at the core surface, although not subject to any particular limitation, has a minimum value of preferably at least 66, more preferably at least 68, and even more preferably at least 70, and a maximum value of preferably not more than 100, more preferably not more than 98, and even more preferably not more than 96.

The value E−C has a lower limit of preferably at least 11, more preferably at least 12, and even more preferably at least 13, and an upper limit of preferably not more than 30, more preferably not more than 29, and even more preferably not more than 28. If the E−C value is too large, a good initial velocity may not be obtained and the durability may worsen. On the other hand, if the E−C value is too small, the spin rate may rise excessively, as a result of which a good distance may not be achieved, and the feel at impact may harden.

No particular limitation is imposed on the method for adjusting the cross-sectional hardness in order to have the core satisfy above formulas (1) to (3), although a core having the desired cross-sectional hardness can be obtained by suitably adjusting the core rubber formulation and the vulcanization temperature and time. For example, a core which satisfies above formulas (1) to (3) can be obtained by adjusting the type and content of the above-described organic peroxide and by adjusting the vulcanization conditions. Specifically, by using both an organic peroxide capable of decomposing at a high temperature and sulfur as rubber compounding ingredients in the core, the desired cross-sectional hardness of this invention can easily be achieved.

In the cover used in this invention, the material making up the outermost layer is exemplified by, but not particularly limited to, known thermoplastic resins or thermoplastic elastomers such as ionomers and polyurethanes.

From the standpoint of lowering the spin rate on full shots with a driver or a middle iron, it is preferable to use an ionomer resin composition as the outermost layer material. In this case, it is desirable that formulation of the ionomer resin composition involve using a mixture of a zinc ion (Zn²⁺)-neutralized ionomer resin and a sodium ion (Na⁺)-neutralized ionomer resin. The mixing ratio therebetween, or zinc ion (Zn²⁺)-neutralized ionomer resin (I)/sodium ion (Na⁺)-neutralized ionomer resin (II), expressed in terms of weight percent, is preferably from 10/90 to 90/10, and more preferably from 15/85 to 85/15. If the (I)/(II) resin ratio does not satisfy the above range, there is a possibility that the rebound of the overall ball becomes smaller, which may make it impossible to obtain the desired flight performance. Moreover, the durability to cracking when repeatedly struck at ordinary temperatures may worsen, and the durability to cracking at low temperatures (sub-zero Celsius) may also worsen.

A commercial product may be used as the above ionomer resin. Illustrative examples include Himilan 1605, Himilan 1557, Himilan 1855 and Himilan AM7331 (all available from DuPont-Mitsui Polychemicals Co., Ltd.).

In addition to the above-described resin components, various additives may be optionally included in the outermost layer material in the invention. Examples of such additives include pigments, dispersants, antioxidants, ultraviolet absorbers, ultraviolet stabilizers, mold release agents, plasticizers, and inorganic fillers (e.g., zinc oxide, barium sulfate, titanium dioxide, tungsten).

Next, the thickness of the outermost layer in this invention, although not subject to any particular limitation, is preferably at least 0.1 mm, more preferably at least 0.3 mm, and even more preferably at least 0.5 mm. The maximum thickness is preferably not more than 1.4 mm, more preferably not more than 1.2 mm, and even more preferably not more than 1.0 mm. If the outermost layer is thicker than the above range, the rebound on W#1 shots may be inadequate or the spin rate may increase, possibly resulting in a poor distance. If the outermost layer is thinner than the above range, the scuff resistance may worsen, or the controllability even by professional golfers and skilled amateurs may be inadequate.

The outermost layer has a material hardness expressed as the Shore D hardness which, although not subject to any particular limitation, is preferably at least 50, more preferably at least 52, and even more preferably at least 54. The maximum value is preferably not more than 70, more preferably not more than 68, and even more preferably not more than 66. At a low Shore D hardness, the ball may be too receptive to spin on full shots, possibly resulting in a poor distance. At a Shore D hardness which is too high, the ball may not be receptive to spin on approach shots, which may result in a poor controllability even by professional golfers and skilled amateurs. It should be noted here that the material hardness of the outermost layer refers to the hardness when the material has been molded into a sheet of a given thickness. The material hardnesses of the intermediate layer and the envelope layer below are also defined in the same way.

In the practice of the invention, by further increasing the number of layers in the ball structure, it is possible to improve the ball performance to a level desired in particular by professional golfers and skilled amateurs. For example, an intermediate layer may be interposed between the above-described core and the above-described outermost layer, although the invention is not limited to this construction.

The intermediate layer has a material hardness expressed as the Shore D hardness which, although not subject to any particular limitation, is preferably at least 35, more preferably at least 40, and even more preferably at least 45. The upper limit is preferably not more than 70, more preferably not more than 66, and even more preferably not more than 60. If the material hardness of the intermediate layer is too low, the ball as a whole may be too receptive to spin on full shots, as a result of which a good distance may not be achieved. On the other hand, if the material hardness of the intermediate layer is too high, the durability to cracking when repeatedly struck may worsen or the feel at impact on shots with a putter or on short approach shots may be too hard.

In addition, it is essential for the intermediate layer and the outermost layer to have a hardness relationship therebetween which satisfies the following formula:

material hardness of intermediate layer<material hardness of outermost layer.

If this formula is not satisfied, a reduction in the spin rate is not achievable, as a result of which the ball has a decreased distance on shots with a driver or a middle iron.

The JIS-C hardness at the surface of a sphere composed of the core encased by the intermediate layer is set to preferably not more than 90. At a JIS-C hardness of more than 90, a reduction in the spin rate may not be achievable, as a result of which the ball may have a decreased distance on shots with a driver or a middle iron.

The thickness of the intermediate layer, although not subject to any particular limitation, is preferably at least 0.5 mm, more preferably at least 0.7 mm, and even more preferably at least 0.9 mm. The upper limit is preferably not more than 2.0 mm, and more preferably not more than 1.7 mm. If the thickness of the intermediate layer is larger than the above range, the spin rate-lowering effect on shots with a W#1 may be inadequate, as a result of which a good distance may not be achieved. If the intermediate layer is too thin, the ball may have a poor durability to cracking when repeatedly struck and a poor durability at low temperatures.

In the present invention, the material of the intermediate layer is not particularly limited, although preferred use may be made of, for example, known ionomeric resins, thermoplastic elastomers and thermoset elastomers. Exemplary thermoplastic elastomers include polyester-based, polyamide-based, polyurethane-based, olefin-based and styrene-based thermoplastic elastomers. In this invention, preferred use may be made of, in particular, a material which includes, as an essential component, a base resin obtained by blending specific amounts of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer, and (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer. In the present invention, by using this material to form at least one intermediate layer, a high rebound on shots with a driver (W#1) can be obtained, enabling a longer distance to be achieved. This material is described in detail below.

The olefin in the above base resin, both in component (a) and in component (b), has a number of carbons which is generally at least 2 but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

Moreover, the unsaturated carboxylic acid ester is preferably a lower alkyl ester of the above unsaturated carboxylic acid. Specific examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) is especially preferred.

The olefin-unsaturated carboxylic acid random copolymer of component (a) and the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer of component (b) (the copolymers in components (a) and (b) are referred to collectively below as “random copolymers”) can each be obtained by random copolymerization of the above components using a known method.

It is recommended that the above random copolymers have unsaturated carboxylic acid contents (acid contents) which are regulated. Here, it is recommended that the content of unsaturated carboxylic acid present in the random copolymer serving as component (a), although not subject to any particular limitation, be set to preferably at least 4 wt %, more preferably at least 6 wt %, even more preferably at least 8 wt %, and most preferably at least 10 wt %. Also, it is recommended that the upper limit, although not subject to any particular limitation, be preferably not more than 30 wt %, more preferably not more than 20 wt %, even more preferably not more than 18 wt %, and most preferably not more than 15 wt %.

Similarly, the content of unsaturated carboxylic acid present in the random copolymer serving as component (b), although not subject to any particular limitation, may be set to preferably at least 4 wt %, more preferably at least 6 wt %, and even more preferably at least 8 wt %. Also, it is recommended that the upper limit, although not subject to any particular limitation, be preferably not more than 15 wt %, more preferably not more than 12 wt %, and even more preferably not more than 10 wt %. If the acid content of the random copolymer is too low, the resilience may decrease, whereas if it is too high, the processability may decrease.

The metal ion neutralization products of the random copolymers of components (a) and (b) may be obtained by neutralizing some of the acid groups on the random copolymer with metal ions. Here, specific examples of metal ions for neutralizing the acid groups include Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. Of these, preferred use may be made of, for example, Na⁺, Li⁺, Zn⁺⁺ and Mg⁺⁺. Moreover, from the standpoint of improving resilience, the use of Na⁺ is recommended. The degree of neutralization of the random copolymer by these metal ions is not subject to any particular limitation. Such neutralization products may be obtained by a known method. For example, use may be made of a method in which neutralization is carried out with a compound such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide of the above-mentioned metal ions.

Sodium ion-neutralized ionomer resins may be advantageously used as the above metal ion neutralization products of the random copolymers to increase the melt flow rate (MFR) of the material. In this way, adjustment of the material to the subsequently described optimal melt flow rate is easy, enabling the moldability to be improved.

Commercially available products may be used as above components (a) and (b). Illustrative examples of the random copolymer in component (a) include Nucrel N1560, Nucrel N1214, Nucrel N1035 and Nucrel AN4221C (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor 5200, Escor 5100 and Escor 5000 (all products of ExxonMobil Chemical). Illustrative examples of the random copolymer in component (b) include Nucrel AN4311, Nucrel AN4318 and Nucrel AN4319 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor ATX325, Escor ATX320 and Escor ATX310 (all products of ExxonMobil Chemical).

Illustrative examples of the metal ion neutralization product of the random copolymer in component (a) include Himilan 1554, Himilan 1557, Himilan 1601, Himilan 1605, Himilan 1706 and Himilan AM7311 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 7930 (E.I. DuPont de Nemours & Co.), and Iotek 3110 and Iotek 4200 (both products of ExxonMobil Chemical). Illustrative examples of the metal ion neutralization product of the random copolymer in component (b) include Himilan 1855, Himilan 1856 and Himilan AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 6320, Surlyn 8320, Surlyn 9320 and Surlyn 8120 (all products of E.I. DuPont de Nemours & Co.), and Iotek 7510 and Iotek 7520 (both products of ExxonMobil Chemical). Sodium-neutralized ionomer resins that are suitable as the metal ion neutralization product of the random copolymer include Himilan 1605, Himilan 1601 and Himilan 1555. Examples of zinc-neutralized ionomer resins preferred as the above metal ion neutralization products of random copolymers include Himilan 1706, Himilan 1557 and Himilan AM7316.

When preparing the base resin, component (a) and component (b) are mixed in a weight ratio of generally between 100:0 and 0:100, preferably between 100:0 and 25:75, more preferably between 100:0 and 50:50, even more preferably between 100:0 and 75:25, and most preferably 100:0. If too little component (a) is included, the molded material obtained therefrom may have a decreased resilience.

The moldability of the base resin can be further improved by, in addition to adjusting the above mixing ratio, also adjusting the mixing ratio between the random copolymers and the metal ion neutralization products of the random copolymers. In this case, it is recommended that the weight ratio of the random copolymers to the metal ion neutralization products of the random copolymers be set to generally between 0:100 and 60:40, preferably between 0:100 and 40:60, more preferably between 0:100 and 20:80, and even more preferably 0:100. The addition of too much random copolymer may lower the uniformity of the pellet composition.

A non-ionomeric thermoplastic elastomer (e) may be included in the base resin so as to enhance even further both the feel of the ball at impact and the rebound. Examples of this component (e) include olefin elastomers, styrene elastomers, polyester elastomers, urethane elastomers and polyamide elastomers. In this invention, to further increase the rebound, it is preferable to use a polyester elastomer or an olefin elastomer. The use of an olefin elastomer composed of a thermoplastic block copolymer which includes crystalline polyethylene blocks as the hard segments is especially preferred.

A commercially available product may be used as component (e). Illustrative examples include Dynaron (JSR Corporation) and the polyester elastomer Hytrel (DuPont-Toray Co., Ltd.).

Component (e) may be included in an amount of more than 0. The upper limit in the amount included per 100 parts by weight of the base resin, although not subject to any particular limitation, is preferably not more than 100 parts by weight, more preferably not more than 60 parts by weight, even more preferably not more than 50 parts by weight, and most preferably not more than 40 parts by weight. Too much component (e) may lower the compatibility of the mixture, possibly resulting in a substantial decline in the durability of the golf ball.

Next, a fatty acid or fatty acid derivative having a molecular weight of at least 228 but not more than 1500 may be added as component (c) to the base resin. Compared with the base resin, this component (c) has a very low molecular weight and, by suitably adjusting the melt viscosity of the mixture, helps in particular to improve the flow properties. Moreover, component (c) includes a relatively high content of acid groups (or derivatives thereof), and is capable of suppressing an excessive loss of resilience.

The molecular weight of the fatty acid or fatty acid derivative of component (c) may be set to at least 228, preferably at least 256, more preferably at least 280, and even more preferably at least 300. The upper limit may be set to not more than 1500, preferably not more than 1000, more preferably not more than 600, and even more preferably not more than 500. If the molecular weight is too low, the heat resistance cannot be improved. On the other hand, if the molecular weight is too high, the flow properties cannot be improved.

Preferred use as the fatty acid or fatty acid derivative of component (c) may likewise be made of, for example, an unsaturated fatty acid (or derivative thereof) containing a double bond or triple bond on the alkyl moiety, or a saturated fatty acid (or derivative thereof) in which the bonds on the alkyl moiety are all single bonds. In either case, it is recommended that the number of carbons on the molecule be preferably at least 18, more preferably at least 20, even more preferably at least 22, and most preferably at least 24. It is recommended that the upper limit be preferably not more than 80, more preferably not more than 60, even more preferably not more than 40, and most preferably not more than 30. Too few carbons may make it impossible to improve the heat resistance and may also make the acid group content so high as to diminish the flow-improving effect on account of interactions with acid groups present in the base resin. On the other hand, too many carbons increases the molecular weight, which may keep a distinct flow-improving effect from appearing.

Specific examples of the fatty acid of component (c) include myristic acid, palmitic acid, stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Preferred use can be made of stearic acid, arachidic acid, behenic acid and lignoceric acid in particular.

The fatty acid derivative of component (c) is exemplified by metallic soaps in which the proton on the acid group of the fatty acid has been replaced with a metal ion. Examples of the metal ion include Na⁺, Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used as component (c) include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred.

Use may also be made of known metallic soap-modified ionomers (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Disclosure WO 98/46671) when using above-described components (a) and/or (b), and component (c).

The amount of component (c) included per 100 parts by weight of the resin components when above components (a), (b) and (e) have been suitably blended may be set to at least 5 parts by weight, preferably at least 10 parts by weight, more preferably at least 20 parts by weight, and even more preferably at least 30 parts by weight. The upper limit in the amount included may be set to not more than 120 parts by weight, preferably not more than 115 parts by weight, more preferably not more than 110 parts by weight, and even more preferably not more than 100 parts by weight. If the amount of component (c) included is too small, the melt viscosity may decrease, lowering the processability. On the other hand, if the amount included is too large, the durability may decrease.

A basic inorganic metal compound capable of neutralizing acid groups in the base resin and in component (c) may be added as component (d). In cases where this component (d) is not included and a metal soap-modified ionomer resin (such as any of the metal soap-modified ionomer resins cited in the above-mentioned patent publications) is used alone, the metallic soap and un-neutralized acid groups present on the ionomer resin undergo exchange reactions during mixture under heating, generating a large amount of fatty acid. Because the fatty acid has a low thermal stability and readily vaporizes during molding, it may cause molding defects. Moreover, if the fatty acid deposits on the surface of the molded material, it may substantially lower paint film adhesion or have other undesirable effects such as lowering the resilience of the resulting molded material.

(1) un-neutralized acid group present on the ionomer resin (2) metallic soap (3) fatty acid X: metal cation

To solve this problem, a basic inorganic metal compound which neutralizes the acid groups present in the base resin and component (c) is included as an essential ingredient (component (d)). By including component (d), the acid groups in the base resin and component (c) are neutralized. Moreover, synergistic effects from the blending of these respective components confer the resin composition with a number of excellent properties; namely, the resin composition has a higher thermal stability and at the same time is imparted with a good moldability, and the resilience as a golf ball-forming material is enhanced.

Illustrative examples of the metal ions used in the basic inorganic metal compound include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Known basic inorganic fillers containing these metal ions may be used as the basic inorganic metal compound. Specific examples include magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. In particular, a hydroxide or a monoxide is recommended. Calcium hydroxide and magnesium oxide, which have a high reactivity with the base resin, are more preferred. Magnesium oxide is especially preferred.

The amount of component (d) included per 100 parts by weight of the resin component may be set to at least 0.1 part by weight, preferably at least 0.5 part by weight, more preferably at least 1 part by weight, and even more preferably at least 1.2 parts by weight. The upper limit in the amount included may be set to not more than 17 parts by weight, preferably not more than 15 parts by weight, more preferably not more than 10 parts by weight, and even more preferably not more than 5 parts by weight. Too little component (d) fails to improve thermal stability and resilience, whereas too much instead lowers the heat resistance of the golf ball-forming material due to the presence of excess basic inorganic metal compound.

By blending specific respective amounts of components (c) and (d) with the resin component, i.e., the base resin containing specific respective amounts of components (a) and (b) in admixture with optional component (e), a material having excellent thermal stability, flow properties and moldability can be obtained, in addition to which the resilience of moldings obtained therefrom can be markedly improved.

It is recommended that the material formulated from specific amounts of the above-described resin component and components (c) and (d) have a high degree of neutralization (i.e., that the material be highly neutralized). Specifically, it is recommended that at least 50 mol %, preferably at least 60 mol %, more preferably at least 70 mol %, and even more preferably at least 80 mol %, of the acid groups in the material be neutralized. Highly neutralizing the acid groups in the material makes it possible to more reliably suppress the exchange reactions that cause trouble when only a base resin and a fatty acid or fatty acid derivative are used as in the above-cited prior art, thus preventing the generation of fatty acid. As a result, the thermal stability is substantially improved and the processability is good, making it possible to obtain molded products of much better resilience than prior-art ionomer resins.

“Degree of neutralization,” as used here, refers to the degree of neutralization of acid groups present within the mixture of the base resin and the fatty acid or fatty acid derivative serving as component (c), and differs from the degree of neutralization of the ionomer resin itself when an ionomer resin is used as the metal ion neutralization product of a random copolymer in the base resin. When a mixture according to the invention having a certain degree of neutralization is compared with an ionomer resin alone having the same degree of neutralization, because the material of the invention contains a very large number of metal ions owing to the inclusion of component (d), the density of ionic crosslinks which contribute to improved resilience is increased, making it possible to confer the molded product with an excellent resilience.

The resin material should preferably have a melt flow rate (MFR) adjusted within a specific range in order to ensure flow properties that are particularly suitable for injection molding, and thus improve moldability. In this case, it is recommended that the melt flow rate, as measured in accordance with JIS-K7210 at a temperature of 190° C. and under a load of 21.18 N (2.16 kgf), be adjusted to preferably at least 0.6 g/10 min, more preferably at least 0.7 g/10 min, even more preferably at least 0.8 g/10 min, and most preferably at least 2 g/10 min. It is recommended that the upper limit be adjusted to preferably not more than 20 g/10 min, more preferably not more than 10 g/10 min, even more preferably not more than 5 g/10 min, and most preferably not more than 3 g/10 min. Too high or low a melt flow rate may result in a substantial decline in processability.

It is preferable to subject the surface of the intermediate layer to abrasion treatment so as to increase adhesion with the outermost layer located on the outside thereof. In addition, following such abrasion treatment, a primer may be applied to the surface. It is also possible to increase adhesion by adding an adhesion reinforcing agent to the intermediate layer material.

In the practice of the invention, the intermediate layer may be formed as a plurality of layers. In cases where a layer (envelope layer) positioned on the innermost side of the intermediate layer and adjacent to the core is provided, there is obtained a multi-piece solid golf ball G having a four-layer construction which, as shown in FIG. 1, is composed of, in order from the inside: a core 1, an envelope layer 2, an intermediate layer 3, and an outermost layer 4 having numerous dimples D on the surface thereof.

The method of manufacturing multi-piece solid golf balls in which the above-described core, intermediate layer and outermost layer are each formed as successive layers is not subject to any particular limitation. Production may be carried out by an ordinary method such as a known injection molding process. By way of illustration, first a core is placed within a given injection mold, following which the intermediate layer material is injection-molded over the core to form an intermediate sphere. Next, the intermediate sphere is placed in another injection mold and the outermost layer material is injection-molded over the intermediate sphere, concurrent with which dimples are molded in the surface of the outermost layer, thereby giving a multi-piece golf ball. Alternatively, instead of the above method in which the materials for the respective layers are injection-molded, use may made of a method in which the intermediate sphere is enclosed by two half-cups that have been molded beforehand into hemispherical shapes, and the resulting assembly is molded under applied heat and pressure.

In the golf ball of the invention, numerous dimples are provided on the surface of the outermost layer for the sake of aerodynamic performance. The number of dimples formed on the outermost layer surface is not subject to any particular limitation. However, to enhance the aerodynamic performance and increase the distance traveled by the ball, the number of dimples is preferably at least 250, more preferably at least 270, even more preferably at least 290, and most preferably at least 300. The maximum number of dimples is preferably not more than 400, more preferably not more than 380, and even more preferably not more than 360.

The golf ball of the invention has a diameter of not less than 42 mm, preferably not less than 42.3 mm, and more preferably not less than 42.6 mm. The upper limit in the diameter is not more than 44 mm, preferably not more than 43.8 mm, more preferably not more than 43.5 mm, and even more preferably not more than 43 mm.

The weight of the golf ball is preferably not less than 44.5 g, more preferably not less than 44.7 g, even more preferably not less than 45.1 g, and most preferably not less than 45.2 g. The upper limit in the weight is not more than 47.0 g, more preferably not more than 46.5 g, and even more preferably not more than 46.0 g.

The golf ball has a deflection when subjected to a compressive load such that the ball deflection when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) is preferably from 10 to 13 mm. If the ball deflection is too small, the feel at impact may harden and the spin rate may rise excessively, as a result of which the desired distance may not be achieved. On the other hand, if the deflection is too large, the ball may not have a good initial velocity and the durability may worsen.

As explained above, the golf ball of this invention provides satisfactory distances on shots with both drivers and middle irons (e.g., I#6) used by professional golfers and skilled amateurs, and also has an excellent durability to cracking when repeatedly struck.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 and 2 Comparative Examples 1 to 6

Golf ball cores were produced by using the rubber formulations in the respective examples of the invention and comparative examples as shown in Table 1 below to prepare core compositions, then molding and vulcanizing the core compositions under the vulcanization conditions in the table. With regard to the core in Comparative Example 6, changes in the hardness profile were effected by impregnating the molded and vulcanized core with an acrylic acid impregnating solution.

TABLE 1 Example Comparative Example 1 2 1 2 3 4 5 6 Polybutadiene A 80 80 80 95 100 100 50 Polybutadiene C 20 50 Polyisoprene rubber 5 Polybutadiene B 20 20 20 80 Peroxide (1) 1.05 1.05 1.05 3.0 0.3 0.6 Peroxide (2) 1.2 0.3 3.0 0.6 Barium sulfate 23.4 21.0 Zinc oxide 17.4 25.6 31.2 4.0 20.5 10.4 7.2 5.0 Antioxidant 0.2 0.2 0.2 0.1 0.1 0.1 0.1 Zinc acrylate 40.3 20.1 6.3 34.0 37.5 34.0 40.0 23.5 Zinc stearate 5 5 5 5 5 5 Sulfur 0.085 0.085 0.085 0.1 0.1 Zinc salt of pentachlorothiophenol 0.4 0.4 0.4 0.6 1.5 1.0 1.5 0.1 Vulcanization temperature (° C.) 155 155 155 155 155 160 160 150 Vulcanization time (min) 21 21 21 13 15 13 13 15 Numbers in the table indicate parts by weight.

Details on the above materials are given below.

-   Polybutadiene A: Available under the trade name “BR730” from JSR     Corporation -   Polybutadiene B: Available under the trade name “BR51” from JSR     Corporation -   Polybutadiene C: Available under the trade name “BR01” from JSR     Corporation -   Polyisoprene rubber: Available under the trade name “IR2200” from     JSR Corporation -   Peroxide (1): Dicumyl peroxide, available under the trade name     “Percumyl D” from NOF Corporation -   Peroxide (2): A mixture of 1,1-di(t-butylperoxy)-cyclohexane and     silica, available under the trade name “Perhexa C-40” from NOF     Corporation -   Antioxidant: 2,2-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Zinc stearate: Available under the trade name “Zinc Stearate G” from     NOF Corporation -   Sulfur: Available under the trade name “Sulfax-5” from Tsurumi     Chemical Industry Co., Ltd.

Next, using the respective resin materials shown in Table 2, in Examples 1 and 2 and Comparative Example 1, an intermediate layer and an outermost layer were formed in this order over the core by injection molding, thereby forming a two-layer cover. In Comparative Examples 2 to 6, an envelope layer, an intermediate layer and an outermost layer were formed in this order over the core by injection molding, thereby forming a three-layer cover. A common dimple configuration I (338 dimples; a plan view of the dimple pattern is shown in FIG. 2) was used for the dimples. The dimples were formed, during injection molding of the outermost layer, by impression with numerous dimple-forming protrusions provided on the spherical surface of the mold cavity.

TABLE 2 (1) (2) (3) (4) (5) (6) (7) (8) HPF 1000 100 Himilan 1605 50 35 Himilan 1557 15 35 Himilan 1706 35 Himilan 1855 15 Himilan AM7331 15 Nucrel AN4319 20 20 Nucrel AN4221C 80 80 Magnesium stearate 60 60 Magnesium oxide 1.7 1.0 Calcium hydroxide 1.5 2.7 Trimethylolpropane 1.1 Polytail H 8 Hytrel 4001 15 T8295 75 75 T8290 37.5 25 25 T8283 62.5 Titanium oxide 3.8 3.5 3.5 Polyethylene wax 1.4 1.5 1.0 Isocyanate com- 7.5 7.5 7.5 pound Numbers in the table indicate parts by weight.

Details on the above materials are given below.

-   HPF 1000: An ionomer available from E.I. DuPont de Nemours & Co. -   Himilan: Ionomers available from DuPont-Mitsui Polychemicals Co.,     Ltd. (AM7331 is a high-stiffness ionomer resin) -   Nucrel: Available from DuPont-Mitsui Polychemicals Co., Ltd. -   Magnesium oxide: Available as “Kyowamag MF150” from Kyowa Chemical     Industry Co., Ltd. -   Calcium hydroxide: Available under the designation CLS-B from     Shiraishi Calcium Kaisha, Ltd. -   Polytail H: A low-molecular-weight polyolefin polyol available from     Mitsubishi Chemical Corporation -   Hytrel: A polyester elastomer available from DuPont-Toray Co., Ltd. -   T8925, T8290, T8283: MDI-PTMG type thermoplastic polyurethanes     available under the trade name “Pandex” from DIC Bayer Polymer -   Polyethylene wax: Available as “Sanwax 161P” from Sanyo Chemical     Industries, Ltd. -   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate

Physical properties such as hardness of the individual layers and the ball, the flight performance (carry), both on shots with a W#1 and on shots with an I#6, and the durability on repeated impact were evaluated according to the criteria described below for the golf balls obtained in each of Examples 1 and 2 and Comparative Examples 1 to 6. The results are presented in Table 3. In (1) to (7) below, measurements were carried out in a 23±1° C. environment.

(1) Golf Ball Deflection

The deflection (mm) of the golf ball when compressed at a temperature of 23±1° C. and a rate of 500 mm/min under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) was measured.

(2) Center Hardness of Core

The core was cut into hemispheres and the cut face was rendered into a flat plane, following which a durometer indenter was pressed perpendicularly against the center thereof and measurement was carried out. The JIS-C (JIS K6301-1975 standard, defined similarly below) hardness value is indicated.

(3) Surface Hardness of Core

A durometer was set perpendicular to a surface portion of the spherical core, and the hardness was measured based on the JIS-C hardness standard. The result was indicated as a JIS-C hardness value.

(4) Cross-Sectional Hardness of Core

The core was cut with a fine cutter and, letting R (mm) be the core radius, B be the JIS-C hardness at a position R/3 mm from the core center, C be the JIS-C hardness at a position R/1.8 mm from the core center, and D be the JIS-C hardness at a position R/1.5 mm from the core center, the JIS-C hardness value at each of these places was measured. The core cross-sectional hardness profiles in the respective working examples and the comparative examples are shown in the graphs in FIGS. 3 to 10.

(5) Material Hardnesses of Intermediate Layer (or Envelope layer)

The resin material for the envelope layer was formed into a sheet having a thickness of 2 mm, and the hardness was measured with a type D durometer in accordance with ASTM-D2240.

(6) Surface Hardness of Intermediate Layer-Covered Sphere

The indenter of a durometer was set substantially perpendicular to the spherical surface of the intermediate layer, and the JIS-C hardness was measured. The surface hardness (JIS-C) of the intermediate layer-covered sphere corresponds to F in Table 3.

(7) Material Hardness of Outermost Layer

The measurement method was the same as in (5) above.

(8) Flight Test

The carry (m) of the ball when struck at a head speed (HS) of 50 m/s with, as the driver (W#1), a TOURSTAGE X-DRIVE 703 (loft angle, 8.5°; manufactured by Bridgestone Sports Co., Ltd.) mounted on a swing robot was measured. The results were rated according to the criteria shown below. The spin rate was the value measured for the ball, immediately after impact, with an apparatus for measuring initial conditions.

Good: Carry was 246 m or more

NG: Carry was less than 246 m

(9) Middle Iron (I#6)

The carry (m) of the ball when struck at a head speed of 44 m/s with, as the middle iron, an X-BLADE CB (a number six iron manufactured by Bridgestone Sports Co., Ltd.) was measured. The results were rated according to the following criteria.

Good: Carry was 150 m or more

NG: Carry was less than 150 m

(10) Durability on Repeated Impact

The durability of the golf ball was evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester fires a golf ball pneumatically and causes it to repeatedly strike two metal plates arranged in parallel. The incident velocity against the metal plates was set at 43 m/s. The number of shots required for the golf ball to crack was measured. The results were rated according to the following criteria.

-   -   Good: Number of shots until cracking occurred was 150 or more     -   NG: Number of shots until cracking occurred was less than 150

TABLE 3 Example Comparative Example 1 2 1 2 3 4 5 6 Core Diameter (mm) 37.3 37.3 37.3 35.4 35.4 35.4 35.4 35.4 A (core center) 58 54 50 60 59 64 59 61 B (R/3 mm from core center) 61 56 52 69 76 73 69 66 C (R/1.8 mm from core center) 63 57 53 69 76 74 72 70 D (R/1.5 mm from core center) 66 59 53 71 75 77 72 72 E (core surface) 88 71 58 80 84 84 89 81 B-A < D-B good good NG NG NG NG NG good C-A 5 3 3 9 17 10 13 9 E-B 27 15 5 11 8 11 20 15 Cover Envelope Material (2) (1) (1) (1) (1) layer Thickness (mm) 1.7 1.7 1.7 1.7 1.7 Sheet D hardness 55 51 51 51 51 (material hardness) Intermediate Material (7) (7) (7) (3) (3) (3) (3) (3) layer Thickness (mm) 1.7 1.7 1.7 1.1 1.1 1.1 1.1 1.1 Sheet D hardness 56 56 56 62 62 62 62 62 (material hardness) F (JIS-C hardness 88 88 88 98 98 98 98 98 at surface of intermediate layer) Outermost Material (8) (8) (6) (4) (4) (4) (4) (4) layer Thickness (mm) 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.8 Sheet D hardness 59 59 57 44 44 44 44 44 (material hardness) Intermediate layer sheet hardness ≧ good good good NG NG NG NG NG Outermost layer sheet hardness Ball Deflection (mm) 10.0 13.0 15.0 8.2 8.4 8.4 8.8 8.9 under 600 kg load Flight W#1, Initial velocity 73 72 71 74 74 74 73 73 HS, 50 m/s (m/s) Spin rate (rpm) 2,252 2,085 2,043 2,507 2,478 2,488 2,453 2,413 Carry (m) 256 250 239 246 245 245 245 246 Rating good good NG good NG NG NG good I#6 Initial velocity 55 55 55 54 54 54 54 54 (m/s) Spin rate (rpm) 5,117 3,803 3,184 6,735 6,623 6,661 6,525 6,205 Carry (m) 163 167 168 148 149 149 149 151 Rating good good good NG NG NG NG good Durability on repeated impact good good NG good NG good good NG (43 m/s)

As shown in Table 3 above, the golf balls obtained in the examples of the invention had an excellent flight performance when struck using a W#1 and an I#6, and also had a good durability. By contrast, the balls obtained in Comparative Examples 1 to 6 either had a poor flight performance when a W#1 or an I#6 was used, or had a poor durability. In Comparative Example 1, the ball was too soft and so the initial velocity was inadequate; the distance was especially poor on shots with a W#1. 

1. A golf ball comprising a core and a cover of at least one layer, wherein the cover comprises an outermost layer and at least one intermediate layer interposed between the outermost layer and the core; the core has a cross-sectional hardness which, letting R (mm) be a radius of the core, A be a JIS-C hardness at a center of the core, B be a JIS-C hardness at a position R/3 mm from the core center, C be a JIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-C hardness at a position R/1.5 mm from the core center, and E be a JIS-C hardness at a surface of the core, satisfies formulas (1) to (3) below: B−A<D−B  (1) C−A≦8  (2) E−B≧15; and  (3) the intermediate layer and the outermost layer have a hardness relationship therebetween which satisfies the following formula: material hardness of intermediate layer<material hardness of outermost layer.
 2. The golf ball of claim 1, wherein at least one intermediate layer is formed of a material obtained by blending: an ionomer resin component comprising (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer mixed with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and (e) a non-ionomeric thermoplastic elastomer in a weight ratio of from 100:0 to 50:50.
 3. The golf ball of claim 1, wherein at least one intermediate layer is formed of a material obtained by blending as essential ingredients: 100 parts by weight of a resin component composed of, in admixture, a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer mixed with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and (e) a non-ionomeric thermoplastic elastomer in a weight ratio between 100:0 and 50:50; (c) from 5 to 120 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and (d) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c).
 4. The golf ball of claim 1, wherein the ball has a deflection, when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf), of from 10 to 13 mm.
 5. The golf ball of claim 1, wherein the core is formed of a rubber composition which includes an organic peroxide having a one-minute half-life temperature of from about 165° C. to about 185° C. and sulfur.
 6. The golf ball of claim 1, wherein the ball has, at a surface of a sphere composed of the core encased by the intermediate layer, a JIS-C hardness of not more than
 90. 7. The golf ball of claim 1, wherein the outermost layer of the cover includes a base resin comprising an ionomer having a material hardness (Shore D) of from 50 to
 70. 8. The golf ball of claim 1, wherein the core has a diameter of from 32 to 41 mm.
 9. The golf ball of claim 1, wherein the core is composed of a single layer. 